@Fabby thanks for the edit, I only was thinking about shrimp, but you're right, all crustaceans turn pink! Good edit :)
– SnakeDocSep 19 at 16:41

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You're welcome! Thanks for the acceptance, favour returned, question upvoted and now it's also a bit more generic so whenever anyone googles for lobster / crab / shrimp turning pink, they'll see your question as the first hit in a few months. Actually let me add red too! ;-)
– FabbySep 19 at 16:44

“Cooking is a form of chemistry” — this really isn’t sufficient reason: Yes, lots of things in cooking are chemical reactions. But not all things are. For example, evaporation when boiling water is a purely physical process, no (relevant) chemical reaction is occurring.
– Konrad RudolphSep 20 at 14:03

@SnakeDoc You continue to misunderstand my point. I’m not saying that chemistry is irrelevant. I’m just saying that it’s not a given that, just because some chemistry is involved in cooking, that every phenomenon in cooking is due to chemistry. To illustrate, your statement is logically equivalent to the famous faulty syllogism “All cats are mortal. Aristotle is mortal. Therefore Aristotle is a cat.”
– Konrad RudolphSep 20 at 16:22

2 Answers
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Crustaceans like shrimp, lobsters, crabs and crayfish have a pigment called astaxanthin in their shells.

Astaxanthin belongs to the terpines class of chemicals of which the carotenoid ¹ class is a subdivision and, in a marine environment, gets produced by an algae that is subsequently consumed by crustaceans (and other animals like salmon, red trout, red sea bream and flamingos² )

As Astaxanthin absorbs blue light, it will appear as its opposing additive colour: a deep red. The more this deep red is diluted, it will subsequently become red, orange or yellow in colour.

While the crustaceans are alive, astaxanthin lies wrapped in the tight embrace of a protein called crustacyanin. The protein holds the pigment so tight, in fact, that it’s flattened and its light-absorption properties are changed. The astaxanthin-crustacyanin complex then winds up giving off a blue-green color. ³

This can be observed if you have aggressive live lobsters you want to cook: just put them in the sink full of water with a glass of white wine added for a few minutes and they will get drunk instantly as they've never had alcohol in their livesrelax and the blue colouring can then be clearly seen at the fronds of their carapace.

The astaxanthin-crustacyanin complex gets:

separated when a crab or lobster is cooked. Crustacyanin is not heat-stable, so introducing it to a boiling pot of water or a grill causes it to relax its bonds with astaxanthin, unravel and let the pigment’s true bold red color shine through. ³

Note ¹: Carrots have given carotenoid its nameNote ²: Eating minuscule shrimp containing this carotenoid is what turns flamingos pink: pink flamingos will be more well-fed than pale flamingos...Note ³:Sourced here

Folks, please try to use comments as intended: for requests for clarification or suggested improvements. We're not entirely anti-fun, and I've left a few, but I've had to delete a lot of things here that went pretty far astray.
– Cascabel♦Sep 21 at 1:46

This is most probably due to the occurrence of a specific carotenoid (Astaxanthin) in their body. This carotenoid (like many others) is

susceptible to enzymatic or nonenzymatic oxidation, which depends on the carotenoid structure, the oxygen availability, enzymes, metals, prooxidants and antioxidants, high temperature, and light exposure